Magatama - I've gotten one of my accounts to pretty high power early on, without paying anything, merely because I was lucky in recent events and got lv3-4 magatama.
Look at growth rates - Stack certain magatama and equipment on ninjas that have the highest growth with that stat.
To Strengthen - Show up at 9tails and always have your strengthens maxed, as well as your group skills.
Be in an active group - Group skills will give you a lot of power, and an active enough group will be able to upgrade skills within the first few days.
Thanks I always reached 2600 - 2650 force at level 28 and I would like to reach it at 2900 as a minimum